Sheet for a thermal conductive substrate, a method for manufacturing the same, a thermal conductive substrate using the sheet and a method for manufacturing the same

ABSTRACT

A thermally conductive substrate having a structure in which inorganic filler for improving the thermal conductivity and thermosetting resin composition are included. The thermosetting resin composition has a flexibility in the not-hardened state, and becomes rigid after hardening. The thermally conductive substrate has excellent thermal radiation characteristics. The method of manufacturing the thermally conductive substrate includes: piling up (a) the thermally conductive sheets comprising 70 to 95 weight parts of an inorganic filler, and 4.9 to 28 weight parts of a thermosetting resin composition, the thermosetting resin composition comprising at least one thermosetting resin, a hardener and a hardening accelerator, and (b) lead frame on which a wiring is formed; thermal pressing the pile; filling the thermally conductive sheet to the surface of the lead frame; hardening the thermosetting resin; cutting excess sections of the thermally conductive substrate; and processing the bending perpendicularly for making a removable electrode.

This application is a division of U.S. Ser. No. 09/495,902, filed Feb.2, 2000, now U.S. Pat. No. 6,355,131, which is a division of U.S. Ser.No. 08/944,799, filed Oct. 6, 1997, now U.S. Pat. No. 6,060,150.

FIELD OF THE INVENTION

The invention relates to a circuit substrate whose thermal radiationproperty is improved by a mixture of resin and inorganic filler. Inparticular, it relates to a high thermal radiation printed wiring boardmade of resin (thermally conductive substrate) for mounting electronicpower devices.

BACKGROUND OF THE INVENTION

Recently, as high performance and miniaturization of the electronicapparatus have been required, high density and high performancesemiconductors have been sought. Consequently, circuit substrates formounting thereof have also been required to be small and of highdensity. As a result, it is important to design circuit substratestaking the thermal radiation property into consideration. A well knowntechnique for improving the thermal radiation property of circuitsubstrates, while using a printed circuit board made of glass-epoxyresin, is to use, a metal base substrate having a metal, for example,aluminum etc. and form a circuit pattern on one face or both faces ofthis metal substrate with an insulating layer interposed in between thecircuit pattern and the metal substrate. Moreover, when higher thermalconductivity is required, the metal base substrate is made of a copperplate, which is directly bonded to a ceramic substrate made of, forexample, alumina or aluminum nitride. For an application requiringrelatively small electric power, a metal base substrate is generallyused. In this case, however, in order to improve the thermal conduction,the insulating layer must be thin. Therefore, as for the substrate ofthin insulating layer, break down voltage is low, and the influence bythe noise, too, is big.

It is difficult for the metal base substrate and ceramic substrate tosatisfy both performance and cost requirements. Recently, an injectionmolded thermally conductive module has been suggested, where athermoplastic resin composition containing inorganic filler isintegrated with the lead frame of an electrode. This injection moldedthermally conductive module has excellent mechanical strength incomparion with a ceramic substrate. However, due to the high viscosityof the thermoplastic resin, it is difficult to injection mold such amodule with a high filler content, and so the thermal radiation propertyof module is poor. In particular, at the time of melting thethermoplastic resin at high temperature and kneading with filler, ifthere is too much filler, the melting viscosity is rapidly increased ina point that not only kneading but also injection molding is madeimpossible. Moreover, the filler serves as an abrasives to abrade themetallic mold, and, thus, reduces the life of the mold. Consequently,the content of the filler is limited, so that only lower thermalconductivity can be obtained as compared with the thermal conductivityof the ceramic substrate.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the above mentionedproblems and to provide a sheet for a thermally conductive substrate inwhich an inorganic filler can be filled in a resin at a high fillerloading to form a thermally conductive module by a simple method, having(a) approximately the same coefficient of the thermal expansion in theplane direction of the substrate as that of a semiconductor, and (b)excellent thermal radiation property; a method for manufacturing theabove mentioned sheet for a thermally conductive substrate; a thermallyconductive substrate using the above mentioned sheet; and a method formanufacturing this thermally conductive substrate.

In order to attain the objects, the sheet for the thermally conductivesubstrate of the present invention is a sheet mixture comprising 70 to95 weight parts of inorganic filler and 5 to 30 weight parts of resincomposition comprising at least thermosetting resin, hardener andhardening accelerator. This sheet mixture has a good flexibility in thehalf hardened state or partially hardened state. (Hereinafter, “B stage”will be used for the half hardened state or partially hardened state.)This sheet mixture of the thermally conductive substrate can be moldedand processed into a predetermined shape due to the flexibility of thesheet. On complete hardening of the resin composition, the substrate canbe made rigid with excellent mechanical strength.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the half hardened state or partially hardenedstate has a viscosity in the range of 10² to 10⁵ (Pa·s). By such apreferred embodiment, excellent flexibility and processing property canbe provided, so that the sheet can be molded and processed into thepredetermined shape. More preferably, the half hardened state orpartially hardened state has a viscosity in the range of 10³ to 10⁴(Pa·s). The viscosity of the sheet herein is measured by the followingmethod: the apparatus used for measuring the elasto-viscosity was a“cone and plate” type dynamic measurement apparatus, MR-500, the productof Rhelogy Co., Ltd.; the sheet was processed into the predeterminedsize and sandwiched between the cone and plate having a diameter of17.97 mm and cone angle of 1.15 deg.; sinusoidal oscillation was givento the sample in the twisting direction; and the difference in thephases of torque which was generated by the sinusoidal oscillation wascalculated. Thus, the viscosity was measured. In the evaluation of theelasto-viscosity of the sheet of the present invention, the sinusoidaloscillation was a sine wave with a frequency of 1 Hz, the strain was 0.1deg., the load was 500 g and the temperature was 25° C.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that 0.1 to 2 weight parts of solvent having aboiling point of not less than 150° C. is further added to 100 weightparts of total weight of inorganic filler and thermosetting resincomposition. By this preferred embodiment, excellent flexibility andprocessing property can be provided.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the solvent having a boiling point of notless than 150° C. is at least one solvent selected from the groupconsisting of ethyl carbitol, butyl carbitol and butyl carbitol acetate.By this preferred embodiment, the processing of the sheet material iseasy, flexibility can be provided to the thermosetting resin at roomtemperature, and the viscosity of the sheet material for molding andprocessing can easily be controlled.

It is preferable in the thermosetting resin composition in the sheet forthe thermally conductive substrate of the present invention to comprise:

1) 0 to 45 weight parts of a first resin that is solid at roomtemperature,

2) 5 to 50 weight parts of a second resin that is liquid at roomtemperature,

3) 4.9 to 45 weight parts of the hardener, and

4) 0.1 to 5 weight parts of the hardening accelerator

when the thermosetting resin composition is 100 weight parts. By such apreferred embodiment, excellent flexibility and processing property canbe provided.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the main component of the thermosetting resinthat is solid at room temperature is one or more components selectedfrom the group consisting of bisphenol A epoxy resin, bisphenol F epoxyresin and liquid phenol resin. By this preferred embodiment, the “Bstage” resin has a long shelf life and the hardened resin has excellentelectrical insulating property and mechanical strength.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the main component of the thermosetting resincomposition is at least one resin selected from the group consisting ofepoxy resin, phenol resin and cyanate resin.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the thermosetting resin composition comprisesbrominated multifunctional epoxy resin as a main component, bisphenol Anovolak resin as a hardener, and imidazole as a hardening accelerator.By such a preferred embodiment, the substrate can be made excellent inflame retardant property, electric insulating property and mechanicalstrength.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the brominated multifunctional epoxy resin bein the range of 60 to 80 weight parts; bisphenol A novolak resin as ahardener be in the range of 18 to 39.9 weight parts, and imidazole as ahardening accelerator be in the range of 0.1 to 2 weight parts.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the inorganic filler is at least one kind offiller selected from the group consisting of Al₂O₃, MgO, BN and AlN,because these fillers are excellent in thermal conductivity.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that at least one additives is selected from thegroup consisting of coupling agent, dispersing agent, coloring agent andtack free agent is further added to the sheet for a thermally conductivesubstrate.

Next, the thermally conductive substrate of the present invention ischaracterized in that when the thermosetting resin component of thethermally conductive substrate sheet is hardened, the coefficient ofthermal expansion is in the range of 8 to 20 ppm /° C. and the thermalconductivity is in the range of 1 to 10 W/mK. In the thermallyconductive substrate, thermal deformation or the like is not generatedand the coefficient of thermal expansion approximates that of asemiconductor.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the flexural strength of the thermallyconductive substrate is not less than 10 Kgf/mm². If the flexuralstrength is within the above mentioned range, practical mechanicalstrength can be obtained. The flexural strength herein is measuredaccording to JIS R-1601 (the testing method of bending strength of fineceramics) in the following manner: test sample is cut in a predeterminedsize; the test sample is placed on two supporting points which arelocated at certain distance; load is applied to the middle point of thetest sample between two supporting points, the maximum bending stresswhen the test sample breaks is measured and this value is defined asflexural strength. This value is also called the three-point bendingstrength.

The dimensions of the test sample are as follows:

Whole Length (Lr): 36 mm

Width (w): 4.0±0.1 mm

Thickness (t): 3.0±0.1 mm

The bending strength is calculated by the following equation:

σ=3PL/2 wt ²

wherein σ denotes the three-point bending strength (kgf/mm²), P denotesthe maximum load when the test piece is broken, L denotes the distancebetween lower supporting points (mm), w denotes the width of the testpiece (mm) and t denotes the thickness of the test piece (mm).

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the flexural strength is in the range of 10to 20 Kgf/mm².

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that a lead frame is further integrated to thethermally conductive substrate, and the thermally conductive substrateis filled to the surface of the lead frame. By such a preferredembodiment, electronic parts can easily be attached to the lead frameand thermal resistance for connecting thermal radiation can beinhibited. Moreover, soldering terminals for connecting a removableelectrode are not required. Instead, the lead frame can be connecteddirectly to an outside signal source, which may be an electrode fortaking current. Thus, reliability by such a preferred embodiment isexcellent.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that a metal substrate for thermal radiation isfurther formed on the face opposite to the face to which the lead frameis adhered to the thermally conductive substrate. By such a preferredembodiment, thermal resistance can be further decreased and themechanical strength is improved.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that a printed circuit board having two or morewiring layers be integrated into a part of the face of the thermallyconductive substrate to which the lead frame is adhered, the thermallyconductive substrate be filled to the surface of the lead frame, and theprinted circuit board comprises two or more wiring layers. By such apreferred embodiment, the control circuit for overcurrent protection ortemperature compensation can be integrated into the substrate. Thus,miniaturization and high density of the apparatus can be realized.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the thermally conductive substrate has athrough hole. The through hole is filled with conductive resincomposition or is plated with copper, and a metallic foil wiring patternis formed and integrated on both sides of the substrate. By such apreferred embodiment, double-sided wiring substrate which is excellentin thermal radiation can be obtained.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that a plurality of the thermally conductivesubstrates are layered and each thermally conductive substrate has athrough hole. The through hole is filled with conductive resincomposition and an internal wiring pattern is composed of conductiveresin composition. In addition, a metallic foil wiring pattern is formedand integrated on both sides of the substrate. By such a preferredembodiment, conductivity between layers of the thermally conductivesubstrate is excellent and internal wiring pattern can be formed.Furthermore, excellent thermal conductivity can be provided.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the metallic foil is a copper foil having athickness of 12 to 200 μm and having faces at least one surface of whichis a rough surface.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the conductive resin composition comprises 70to 95 weight parts of at least one metallic powder selected from thegroup consisting of silver, copper and nickel: and 5 to 30 weight partsof thermosetting resin and hardener.

It is preferable in the sheet for the thermally conductive substrate ofthe present invention that the inorganic filler has an average particlediameter of 0.1 to 100 μm.

The first method of manufacturing the sheet for the thermally conductivesubstrate of the present invention comprises the steps of: forming aslurry mixture comprising 70 to 95 weight parts of an inorganic filler,4.9 to 28 weight parts of a thermosetting resin composition and 0.1 to 2weight parts of a solvent having a boiling point of not less than 150°C. and solvent having a boiling point not more than 100° C.; forming theslurry mixture into a film having a desired thickness; and drying thesolvent having a boiling point of not more than 100° C. of the filmslurry.

The second method of manufacturing the sheet for the thermallyconductive substrate of the present invention comprises the steps of:forming a slurry mixture comprising 70 to 95 weight parts of inorganicfiller, 5 to 30 weight parts of thermosetting resin mixture comprising asolid of thermosetting resin that is solid at room temperature and aliquid thermosetting resin that is liquid at room temperature andsolvent having a boiling point not more than 100° C.; forming the slurrymixture into a film having a desired thickness; and drying only thesolvent having a boiling point of not more than 100° C. of the filmslurry.

It is preferable in the second manufacturing method that thethermosetting resin mixture in the sheet for thermally conductivesubstrate made according to the second method, comprises:

1) 0 to 45 weight parts of resin that is solid at room temperature,

2) 5 to 50 weight parts of resin that is liquid at room temperature,

3) 4.9 to 45 weight parts of hardener, and

4) 0.1 to 5 weight parts of hardening accelerator

when the total weight of the solid thermosetting resin and the liquidthermosetting resin 100 is weight parts.

It is further preferable in the second manufacturing method that themain component of the solid thermosetting resin is one or morecomponents selected from the group consisting of bisphenol A epoxyresin, bisphenol F epoxy resin and liquid phenol resin.

It is preferable in the first and second manufacturing methods that thethermosetting resin mixture comprises a brominated multifunctional epoxyresin as a main component, a bisphenol A novolak resin as a hardener,and an imidazole as a hardening accelerator.

It is preferable in the first and second manufacturing methods that thesheet for a thermally conductive substrate comprises a brominatedmultifunctional epoxy resin in the range of 60 to 80 weight parts; abisphenol A novolak resin as a hardener in the range of 18 to 39.9weight parts, and an imidazole as a hardening accelerator in the rangeof 0.1 to 2 weight parts.

It is preferable in the first manufacturing method that the solventhaving a boiling point of not less than 150° C. is at least one solventselected from the group consisting of ethyl carbitol, butyl carbitol andbutyl carbitol acetate.

It is preferable in the first and second manufacturing methods that thesolvent having a boiling point of not more than 100° C. is one solventselected from the group consisting of methyl ethyl ketone, isopropanoland toluene.

It is preferable in the first and second manufacturing methods that anadditive selected from the group consisting of coupling agent,dispersing agent, coloring agent and tack free agent is further added tothe sheet for a thermally conductive substrate.

It is preferable in the first and second manufacturing methods that thefilm forming method is at least one method selected from the groupconsisting of doctor blade method, coater method, and injection moldingmethod.

The third method for manufacturing the thermally conductive substrate ofthe present invention comprises the steps of: piling up a lead frame ona face of the sheet for the thermally conductive substrate manufacturedby the first manufacturing method; molding the sheet at a temperaturebelow the hardening temperature of the thermosetting resin compositionand at a pressure in the range of 10 to 200 Kg/cm²; filling the sheetand integrating to the surface of the lead frame; and hardening thethermosetting resin by thermal pressing at the pressure in the range of0 to 200 Kg/cm².

It is preferable in the third manufacturing method that a metalsubstrate for thermal radiation is further formed on the face oppositeto the face to which the lead frame is adhered to the thermallyconductive substrate.

Moreover, the third method for manufacturing the thermally conductivesubstrate of the present invention comprises the steps of: placing thelead frame and a printed circuit board having two or more wiring layerson the sheet for the thermally conductive substrate manufactured by thefirst manufacturing method in a way in which the lead frame and theprinted circuit board are not overlapped; molding the sheet at thetemperature below the hardening temperature of the thermosetting resincomposition and at the pressure in the range of 10 to 200 Kg/cm²;filling the sheet and integrating to the surface of the lead frame andthe printed circuit board having two or more wiring layers; andhardening the thermosetting resin by thermal pressing at the pressure of0 to 200 Kg/cm².

Moreover, the third method for manufacturing the thermally conductivesubstrate of the present invention comprises a series of steps of:processing through holes on the sheet for the thermally conductivesubstrate manufactured by the first manufacturing method; filling aconductive resin composition into the through holes; piling up themetallic foil on both sides of the sheet into which the conductive resincomposition is filled in the through holes; hardening the thermosettingresin of the sheet by thermal pressing at the pressure of 10 to 200Kg/cm²; and forming wiring pattern by processing the metallic foil.

Moreover, the method for manufacturing the thermally conductivesubstrate of the present invention comprises the steps of: piling up ametallic foil on the both sides of the sheet for the thermallyconductive substrate manufactured by the first manufacturing method;hardening the thermosetting resin of the sheet of thermally conductivesubstrate by thermal pressing at the pressure of 10 to 200 Kg/cm²;processing through holes on the hardened the thermally conductive sheet;conducting a copper plating on the entire surface of the sheet on whichthrough holes are processed; and forming a wiring pattern by processingthe metallic foil and the copper plating layer.

Moreover, the third method for manufacturing the thermally conductivesubstrate of the present invention comprises the steps of: preparing adesired number of thermally conductive substrates by the firstmanufacturing method; processing through holes at desired locations oneach of the sheets; filling a conductive resin composition into thethrough holes; forming a wiring pattern on one surface of the filledsheet by using the conductive resin composition; piling up each of thesheet having the wiring pattern in a way in which the surface having thewiring pattern is adjusted to face upward and the sheet on which onlythe conductive resin composition is filled to the through hole isadjusted to be the top face to form a pile; piling up metallic foil onboth sides of the pile; hardening the thermosetting resin of the sheetfor the thermally conductive substrate by thermal pressing at thepressure of 10 to 200 Kg/cm²; and forming a wiring pattern by processingthe metallic foil.

It is in the third manufacturing method that the through holes areprocessed by the method selected from the group consisting of laser beamprocess, drilling process and punching process.

It is in the third manufacturing method that the metallic foil is acopper foil having a thickness of 12 to 200 μm and having faces at leastone surface of which is a rough surface.

It is in the third manufacturing method that the conductive resincomposition comprises 70 to 95 weight parts of at least one metallicpowder selected from the group consisting of silver, copper and nickel;and 5 to 30 weight parts of thermosetting resin and hardener.

It is in this third manufacturing method that the temperature for thethermal pressing is in the range of 170 to 260° C.

As mentioned above, according to the present invention, high thermalradiation printed circuit wiring board for mounting electronic powerdevices can be made of the thermally conductive substrate by shaping andhardening the thermally conductive sheet into a desired shape. Shapingis possible due to the flexibility of the thermally conductive substratesheet, hardening makes the thermally conductive substrate rigid.

Moreover, according to the present invention, thermally conductivesubstrate can be manufactured efficiently and reasonably.

The first embodiment of the present invention basically relates to athermally conductive sheet having flexibility, where an inorganic filleris added into a thermosetting resin in the not-hardened state at highdensity; the coefficient of thermal expansion in the plane direction isapproximately the same as that of Si semiconductor; and high thermalconductivity is provided. In the thermally conductive sheet of thepresent invention, a high boiling point solvent is added into thethermosetting resin composition, or a thermosetting resin mixturecontaining a solid resin that is solid at room temperature and a liquidthermosetting resin that is liquid at room temperature, and films areformed by using a low boiling point solvent for mixing with inorganicfiller. Consequently, in the thermally conductive sheet of the presentinvention, inorganic filler can be added at a high filler loading.Furthermore, the flexibility of the thermosetting resin of the thermallyconductive sheet is manufactured in the not-hardened state, and, thus,molding the thermally conductive sheet into a desired shape at a lowtemperature and at a low pressure is possible. In addition, thethermally conductive substrate can be made rigid by hardening thethermosetting resin by thermal pressing. Also, a thermally conductivesubstrate on which a semiconductor can be simply and directly mountedcan be obtained by the use of this thermally conductive sheet which isflexible.

The second embodiment of the present invention relates to a thermallyconductive substrate on which a semiconductor having thermal radiationproperty can directly be mounted by using the thermally conductivesheet; piling up a lead frame; and hardening the thermally conductivesheet by means of thermal pressing to integrate with the lead frame.

Moreover, the third embodiment of the present invention relates to adoubled-sided thermally conductive substrate having high thermalconductivity, which permits electrical conductivity on both sides byforming through holes on the thermally conductive sheet, filling thethorough holes with the conductive resin composition and formingmetallic foil patterns on both sides of the sheet.

Moreover, the fourth embodiment of the present invention relates to ahigh thermally conductive double-sided substrate which permits electricconductivity by copper plating to the through holes of the thirdembodiment.

Moreover, the fifth embodiment of the present invention relates to athermally conductive substrate (a multi-layered substrate) having amulti-layered circuit structure in which a plurality of the thermallyconductive sheets are used, the through holes to which conductive resincomposition is filled are formed, wiring pattern is formed on one sideof the thermally conductive sheet, and a plurality of the thermallyconductive sheets are piled up.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view showing a structure of the thermallyconductive sheet of one embodiment of the present invention.

FIGS. 2A to 2E are cross sectional views showing each step of amanufacturing process of the thermally conductive substrate which ismanufactured by using the thermally conductive sheet of one embodimentof the present invention.

FIG. 3 is a cross sectional view of the thermally conductive substrateon which the thermal radiation metal substrate is further formed on theface opposite to face the lead frame is adhered to the thermallyconductive substrate manufactured by the process according to FIG. 2.

FIGS. 4A to 4F are cross sectional views showing each step of amanufacturing process of the thermally conductive substrate which ismanufactured by using the thermally conductive sheet of one embodimentof the present invention.

FIG. 5 is a cross sectional view showing a process for manufacturing thethermally conductive multi-layered wiring substrate of one embodiment ofthe present invention.

FIGS. 6A to 6J are cross sectional views showing each step of amanufacturing process of the thermally conductive multi-layered wiringsubstrate of one embodiment of the present invention.

FIGS. 7A and 7B are cross sectional views showing each step of amanufacturing process of the thermally conductive substrate of anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the thermally conductive substrate (single-sided wiringsubstrate, double-sided wiring substrate, multi-layered wiringsubstrate) for mounting bare chip of one embodiment of the presentinvention will be explained by referring to figures.

FIG. 1 is a cross sectional view showing a structure of the thermallyconductive sheet of one embodiment of the present invention. In FIG. 1,a thermally conductive sheet 100 is formed on a tack free film 101. Theforming method includes: preparing the slurry mixture which comprises atleast one inorganic filler, thermosetting resin composition, a solventhaving a boiling point of not less than 150° C. and a solvent having aboiling point of not more than 100° C.; and forming the mixture into afilm on the tack free film 101. The film forming method can be, forexample, a doctor blade method, a coater method and an injection moldingmethod can be employed. A thermally conductive sheet having flexibilitycan be obtained by drying only the solvent having a boiling point of notmore than 100° C. of the film slurry.

Moreover, similarly, a thermally conductive sheet having flexibility canbe obtained by the process comprising the steps of: preparing the slurrymixture which comprises at least one inorganic filler, a thermosettingresin composition that is solid at room temperature, and a solventhaving a boiling point of not more than 100° C.; forming the slurrymixture into a film on the tack free film 101, similar to the above; anddrying the solvent.

The examples of the thermosetting resin include, epoxy resin, phenolresin and cyanate resin. Moreover, the examples of the inorganic fillerinclude Al₂O₃, MgO, BN, and AlN. The examples of the solvent having aboiling point of not less than 150° C. include ethyl carbitol, butylcarbitol and butyl carbitol acetate.

Moreover, the examples of the thermosetting resin that is liquid at roomtemperature include epoxy resin such as bisphenol A epoxy resin,bisphenol F epoxy resin and liquid state phenol resin.

In addition, the examples of the solvent having a boiling point of notmore than 100° C. include methyl ethyl ketone, isopropanol and toluene.Moreover, if necessary, coupling agent, dispersing agent, coloring agentand tack free agent can be added as an additive into the thermallyconductive sheet composition.

Moreover, as mentioned above, the half hardened or partially hardenedsheet for the thermally conductive substrate having a moderatedviscosity (10² to 10⁵ Pa·s) can be obtained by adding the solvent havinga boiling point of not less than 150° C. or adding the thermosettingresin that is liquid at room temperature, and drying the solvent havinga boiling point of not more than 100° C. If the viscosity is not morethan 10² Pa·s, the adhesion of the sheet is so strong that it isdifficult to be peeled apart from the tack free film and furthermorechanging the shapes after the process is large and the operationefficiency is bad. It is preferable that the viscosity is in the rangeof 10³ to 10⁴ Pa·s in the view of the operation efficiency andprocessing property.

Since high filler loading (ie., high filler content) in the thermallyconductive substrate using this thermally conductive sheet is possiblein the present invention, the coefficient of thermal expansion of thesubstrate can be made to be approximately the same as that of asemiconductor, and furthermore the substrate can be made to haveexcellent thermal radiation.

FIGS. 2A to 2E are cross sectional views showing each step of amanufacturing process of the thermally conductive substrate manufacturedby using the thermally conductive sheet 100. In FIG. 2A, numeral 200denotes the thermally conductive sheet manufactured by the above method;201 of FIG. 2B denotes a lead frame forming a wiring. The lead frame 201can be obtained by punching a copper plate into a desired shape, or canbe formed by an etching method. A processed lead frame whose surface isplated with nickel prevent oxidation of copper is generally used.

FIG. 2C shows a structure in which the lead frame 201 and the thermallyconductive sheet 200 are piled up.

FIG. 2D shows a structure in which the lead frame and thermallyconductive sheet are thermally pressed. Then the thermally conductivesheet is filled to the surface of the lead frame by using theflexibility of the thermally conductive sheet. Finally, thethermosetting resin in the thermally conductive sheet is hardened. Then,FIG. 2E shows the hardened thermally conductive substrate in which theportion except the necessary portion of the lead frame of the thermallyconductive substrate is cut. In FIG. 2E, the hardened thermallyconductive substrate is bent perpendicularly so as to form a removableelectrode. Thus, as described above, a thermally conductive substrate ismanufactured. Subsequently, steps of mounting parts by soldering orfilling insulating resin are carried out, but they are not importantherein and omitted.

FIG. 3 shows a structure in which the thermal radiation metal substrate302 is further formed on the face opposite to the portion where the leadframe is adhered to the thermally conductive substrate manufactured bythe steps illustrated in FIG. 2.

FIGS. 4A to 4F show the method for forming a thermally conductivesubstrate having a double-sided wiring, which is different from theabove mentioned method. FIG. 4A shows the thermally conductive sheet 400formed on the tack free film 401. In FIG. 4B, the through holes 402 isformed from the side of the tack free film 401 of the thermallyconductive sheet 400. The formation of the through holes can beconducted by a laser processing method using carbon dioxide, excimer orthe like, or by a metal molding process, or furthermore, by drilling.Punching by using laser beam is preferred. because punching holes at afine pitch is possible and scrapings are not generated. In FIG. 4C, theconductive resin composition 403 is filled into the through hole 402. Asa conductive resin composition, for example, a conductive paste formedby mixing copper powder, epoxy resin and hardener of epoxy resin can beused. In FIG. 4D, the metallic foils 404 are further piled up on bothsides of the thermally conductive sheet. FIG. 4D is thermally pressed inthis state, and the thermally conductive sheet is hardened as shown inFIG. 4E. Finally, the metallic foil applied onto both sides areprocessed as shown in FIG. 4F, and thereby the wiring pattern 405 can beobtained. Thus, the thermally conductive substrate having wiringpatterns on both sides of the thermally conductive sheet can beobtained. At this time, the lead frame can be used instead of themetallic foil. In this case, the last step, namely, the step for formingwiring pattern can be omitted.

FIG. 5 is a cross sectional view of the thermally conductive substrate,where the method of electrically connecting both sides of the thermallyconductive substrate manufactured by the process of FIG. 4 is conductednot by the use of conductive resin composition but by processing throughholes after hardening by thermal pressing, followed by connecting theinside layers by the copper plating method. Numeral 501 denotes a copperplating layer formed on the inside surface of the through hole; 502denotes a wiring pattern; and 500 denotes the thermally conductivesubstrate wherein the thermally conductive sheet is hardened.

FIG. 6 is a cross sectional view showing each step of a manufacturingprocess of a thermally conductive multi-layered wiring substrate of oneembodiment of the present invention. FIGS. 6A to 6C are the same as thethermally conductive sheet shown in FIG. 4 where through holes areprocessed on the thermally conductive sheet and a conductive resincomposition is filled into the through holes. FIGS. 6D, 6F and 6G showthermally conductive sheets into which the conductive resin composition603 is filled and the conductive resin composition 603 is further usedon one side thereof to produce wiring pattern 604. The method forforming the wiring pattern can be a screen printing method or a copperplate offset printing method or the like. In FIG. 6E, the wiring patternby the conductive resin composition is not formed.

FIG. 6H is a pile where the thermally conductive sheets shown in FIGS.6E to 6G are piled up as shown in the figure, and a metallic foil 605 isfurther piled up on the both sides of the pile. FIG. 6I shows astructure in which each thermally conductive sheet is laminated,hardened and adhered by thermal pressing. FIG. 6J shows a structure inwhich the wiring pattern of the top layer 606 is finally formed. Theformation of the wiring pattern herein is carried out by an etchingmethod. The etching method is wet etching, where ferric chloride is usedas the etching reagent. Thus, a high density thermally conductivesubstrate having a multi-layered wiring structure can be obtained.

Moreover, herein, in manufacturing the printed circuit board, there aresteps of applying soldering resist, printing letters or marks andpunching holes for inserting parts. For these steps, however, anyconventional technique can be employed, and they are not importantherein and therefore omitted.

FIGS. 7A and 7B are cross sectional views showing each step of amanufacturing process of the thermally conductive substrate manufacturedby using the thermally conductive sheet 700. In FIG. 7A, numeral 700denotes the thermally conductive sheet manufactured by the above method;701 denotes a lead frame for forming wiring. The lead frame 701 can beobtained by punching a copper plate into a desired shape, or can beformed by an etching method. A processed lead frame whose surface isplated with nickel preventing oxidation of copper is generally used.Numeral 702 denotes the printed wiring circuit having two or more wiringlayers and it has a via 704 for electrically connecting between thewiring pattern 703 and the layers.

FIG. 7B shows a structure in which the lead frame 701, thermallyconductive sheet 700 and the printed wiring circuit 702 having twolayers or more are thermally pressed; then the thermally conductivesheet is filled to the surface of the lead frame by using theflexibility of the thermally conductive sheet; and furthermore, thethermosetting resin in the thermally conductive sheet is hardened. Then,as in FIG. 2E, the hardened thermally conductive substrate in which theportion except the necessary portion of the lead frame of the thermallyconductive substrate is cut, the hardened thermally conductive substrateis bent perpendicularly so as to form a removable electrode. Thus, athermally conductive substrate is manufactured. Subsequently, steps ofmounting parts by soldering or filling insulating resin are carried out,but they are not important herein and omitted.

Hereinafter, the present invention will be explained by referring toExamples.

EXAMPLE 1

In the formation of the thermally conductive sheet of the presentinvention; inorganic filler, thermosetting resin and solvent were mixed,and alumina balls were further added into the above mixture so as toobtain a sufficient dispersion. The compositions of the thermallyconductive sheet of this Example are shown in Table 1.

TABLE 1 Thermosetting Solvent having a resin boiling point of SheetInorganic (including not more than after Experi- Filler hardener) 150°C. dried ment Vol. Vol. Vol. Other additives Viscosity No. Name (wt %)Name (wt %) Name (wt %) *1 *2 *3 (Pa · s) 1a Al₂O₃ 60 Epoxy 36 Butyl 4 —— — 1.5 × 10² resin calbitol 1b Al₂O₃ 70 Epoxy 28 acetate 2 — — — 3.3 ×10³ resin (BCA) 1c Al₂O₃ 80 Epoxy 18 2 — — — 2.6 × 10⁴ resin 1d Al₂O₃ 90Epoxy 9.5 0.5 — — — 8.1 × 10⁴ resin 1e Al₂O₃ 95 Epoxy 4.9 0.1 — — — 1.3× 10⁵ Resin *1: coloring agent *2: coupling agent *3: dispersing agent

Table 1 shows an evaluation of the performance of the thermallyconductive sheet when the content of Al₂O₃ as an inorganic filler ischanged. As Al₂O₃, “AL-33” having a particle diameter of 12 μm on theaverage, the product of Sumitomo Chemical Company Limited was used; andas an epoxy resin, the epoxy resin comprising the following compositionwas used: 1) thermosetting resin main agent: 65 weight parts ofbrominated multifunctional epoxy resin (5049-B-70, the product ofYuka-shell epoxy Co., Ltd.); 2) hardener: 34.4 weight parts of bisphenolA novolak resin (152, the product of Yuka-shell epoxy Co., Ltd.); and 3)hardening acceleratoer: 0.6 weight parts of imidazol (EMI-12, theproduct of Yuka-shell epoxy Co., Ltd.). This resin composition was in asolid state and it was softened to a paste-like consistence by addingmethyl ethyl ketone. The content in the state of solid was 70%.

First, the resin compositions in Table 1 were weighed. Then, methylethyl ketone solvent, having a boiling point of not more than 100° C.for adjusting the viscosity, was added to the compositions until theviscosity of the slurry became about 20 Pa·s. Subsequently, the aluminaballs were added and mixed thereof in a pot with a rotating at a speedof 500 rpm for 48 hours. At this time, the low boiling point solvent wasused so as to adjust the viscosity of the alumina ball filled slurry.Maintaining a low slurry-like viscosity by this slurry is important foradding an inorganic filler in a high concentration in the slurry.However, the low boiling point solvent was volatilized in the followingdrying step. Since no low boiling point solvent remained in thethermally conductive sheet composition, it is not included in Table 1.Next, a polyethylene terephthalate sheet having a thickness of 75 μm wasprepared as the tack free surface and the above mentioned slurry wasspread out into a film by the doctor blade method with a gap ofapproximately 1.4 mm. Then, methyl ethyl ketone in the above mentionedfilm was dried by allowing the film to stand at 100° C. for an hour.Thereby, as shown in Table 1, the flexible thermally conductive sheet(the thickness was 750 μm) having a moderate viscosity was obtained.

From the thermally conductive sheet manufactured like this, the tackfree film of polyethylene terephthalate film was peeled apart. Then, thethermally conductive sheet was again covered with a thermal resistancetack free film (PPS: polyphenylene sulfite having a thickness of 75 μm),and was hardened at the temperature of 200° C. and at the pressure of 50Kg/cm². The PPS tack free film was peeled apart and the thermallyconductive sheet was processed into a predetermined shape and size. Thethermal conductivity, coefficient of thermal expansion, break downvoltage and flexural strength were measured. The results are shown inTable 2.

TABLE 2 Evaluation of thermally conductive substrate Break Experi-Thermal Thermal Down Flexural ment Conductivity Expansion VoltageStrength No (W/mK) (ppm/° C.) (kV/mm) (Kg/mm²) 1a 1.1 28 15 9.5 1b 1.224 14 12.3 1c 1.9 18 15 15.5 1d 3.5 10 12 18.8 1e 4.1  8  9 13.1

The thermal conductivity was defined by calculating the temperaturetransmitted from one surface to another surface of the sample that wascut into 10 mm in size, when the sample was heated by bringing it intocontact with a heater. Similarly, the break down voltage by AC voltageshown in Table 2 was defined by measuring the break down voltage in thedirection of the thickness of the thermally conductive substrate andcalculating the value per a unit thickness. The break down voltage isaffected by the adhesion between the thermosetting resin and theinorganic filler in the thermally conductive substrate. In other words,if the wettability of the inorganic filler and the thermosetting resinwas bad, micro gaps were generated between them. As a result, thestrength of the substrate and break down voltage are deteriorated. Ingeneral, the break down voltage of a resin alone is approximately 15KV/mm. If the break down voltage is not less than 10 KV/mm, it is judgedthat the adhesion between the thermosetting resin and the inorganicfiller is excellent.

From the results in Tables 1 and 2, the thermally conductive substrateobtained by the thermally conductive sheet manufactured by the abovementioned method had about 20 times as much thermal conductivity as theconventional glass epoxy substrate, and not less than 2 times as high aperformance as the thermally conductive sheet manufactured by theconventional injection molding method. In addition, as to thecoefficient of thermal expansion, when the thermally conductive sheetcontained not less than 90 wt. % of Al₂O₃, the coefficient of thermalexpansion was similar to that of a silicon semiconductor. Moreover, theflexural strength of the substrate was not less than 15 Kg/mm²,exhibiting sufficient strength as substrate. Therefore, the thermallyconductive substrate of the present invention is promising as asubstrate for a flip chip on which a semiconductor is directly mounted.

Then, the performance was evaluated when the type of inorganic fillerwas changed. The compositions are shown in Table 3 and the evaluationresults are shown in Table 4.

TABLE 3 Thermosetting Solvent having a resin boiling point of SheetInorganic (including not more than after Experi- Filler hardener) 150°C. dried ment Vol. Vol. Vol. Other additives Viscosity No. Name (wt %)Name (wt %) Name (wt %) *1 *2 *3 (Pa · s) 1f Al₂O₃ 91 Epoxy  8 Butyl 0.50.3 0.2 — 6.1 × 10⁴ resin calbitol 1g AlN 85 Epoxy 14 acetate 0.5 0.30.2 — 1.6 × 10⁴ resin (BCA) 1h AlN 90 Epoxy  9 0.5 0.3 — 0.2 5.8 × 10⁴resin 1I BN 80 Epoxy 19 0.5 0.3 0.2 — 7.1 × 10³ resin 1j MgO 87 Epoxy 120.5 0.3 0.2 — 6.4 × 10⁴ resin *1: coloring agent (wt. %) *2: couplingagent (wt. %) *3: dispersing agent (wt. %)

TABLE 4 Evaluation of Thermal Conductive Substrate Thermal Thermal BreakDown Flexural Experiment Conductivity Expansion Voltage Strength No.(W/mK) (ppm/° C.) (kV/mm) (Kg/mm²) 1f 3.7 9 11 18.5 1g 4.0 11 14 153 1h7.4 7.5 12 13.6 1I 3.5 12 15 10.9 1j 4.2 19 10 12.0

As is apparent from Tables 3 and 4, if a powder other than Al₂O₃, forexample, AlN, MgO, BN (approximately 7 to 12 μm) was used as theinorganic filler, the performance peculiar to the inorganic filler wasexhibited. In other words, if AlN having excellent thermal conductivitywas used, then the thermal conductivity similar to that of the ceramicsubstrate was obtained (Example 1h). Moreover, in a case where BN wasadded, then high thermal conductivity and low thermal expansion propertywas obtained as shown in Example 1i. At this time, the additives contentwas determined in a way in which a suitable state could be obtained inaccordance with the density and dispersion of inorganic fillers. Moreinorganic fillers can be added by adding dispersing agents such as AlN.Moreover, the thermally conductive substrate having a sufficient thermalradiation property could be obtained by coloring the thermallyconductive sheet. Moreover, as mentioned above, the addition of silanecoupling agent for improving the adhesion between the organic filler andthe thermosetting resin also improves the break down voltagecharacteristics of the thermally conductive sheet.

In Table 5, the performance of the thermally conductive sheet wasevaluated in a case where Al₂O₃ was used as the inorganic filler andresin that is liquid at room temperature was added for providingflexibility. As Al₂O₃, “AL-33” (the product of Sumitomo Chemical CompanyLimited) having an average particle diameter of 12 μm, was used; and anepoxy resin was obtained by substituting a part of NVR-1010 containing ahardener (the product of Japan REC Co., Ltd.) by liquid resin shown inTable 5.

TABLE 5 Thermosetting Thermosetting resin that is resin that isInorganic Solid at room liquid at room Other Sheet Experi- FillerTemperature Temperature Hardener additives after ment Vol. Vol. Vol.Vol. Vol. dried No. Name (wt %) Name (wt %) Name (wt %) Name (wt %) Name(wt %) (Pa · s) 1k Al₂O₃ 89.5 Epoxy 9 bis F 1 Sl-100 0.2 Raven 0.3 3.1 ×10⁵ resin 1060 1l Al₂O₃ 89.5 Epoxy 8 bis F 2 Sl-100 0.2 Raven 0.3 4.3 ×10⁴ resin 1060 1m Al₂O₃ 89.5 Epoxy 6 bis F 4 Sl-100 0.2 Raven 0.3 4.4 ×10³ resin 1060 1n Al₂O₃ 89.5 Epoxy 4 bis F 6 Sl-100 0.2 Raven 0.3 2.1 ×10² resin 1060 1o Al₂O₃ 89.5 Epoxy 6 bis A 4 Sl-100 0.2 Raven 0.3 6.7 ×10⁴ resin 1060 1p Al₂O₃ 89.5 Epoxy 6 phenol 4 Sl-100 0.2 Raven 0.3 3.9 ×10³ resin 1060 Bis F: bisphenol F epoxy resin (806, the product ofYuka-Shell Epoxy) Bis A: bisphenol A epoxy resin Phenol: liquid statephenol resin (110, the product of Cemedyne Co., Ltd.) Hardener Sl-110:San-aid (the product of Sanshin Kagaku Co., Ltd.) Coloring agent: carbonblack (Raven 1060, the product of Colombia Carbon Japan Co., Ltd)

First, the compositions in Table 5 were weighed. Then, methyl ethylketone solvent, having a boiling point of not more than 100° C. foradjusting the viscosity, was added to the compositions until theviscosity of the slurry became about 20 Pa·s. Subsequently, the aluminaballs were added and mixed thereof in a pot with a mixing devicerotating at a speed of 500 rpm for 48 hours. At this time, the lowboiling point solvent was used to adjust the viscosity of the aluminaballs filled slurry. Maintaining a low slurry-like viscosity by theslurry is important for adding an inorganic filler in a highconcentration in the slurry. However, the low boiling point solvent wasvolatilized in the following drying step. Since no low boiling pointsolvent remained in the thermally conductive sheet composition, it isnot included in Table 1. Next, a polyethylene terephthalate sheet havingthe thickness of 75 μm was prepared as the tack free surface and theabove mentioned slurry was spread out into a film by the doctor blademethod with a gap of approximately 1.4 mm. Then, methyl ethyl ketone inthe above mentioned film was dried by allowing to stand at 100° C. foran hour. Thereby, as shown in Table 5, the flexible thermally conductivesheet (the thickness was 750 μm) having a moderate viscosity wasobtained by adding a resin that is liquid at room temperature.

From the thermally conductive sheet manufactured like this, the tackfree film of polyethylene terephthalate film was peeled apart. Then, thethermally conductive sheet was again covered with a thermal resistancetack free film (PPS: polyphenylene sulfite having a thickness of 75 μm),and was hardened at the temperature of 200° C. and at the pressure of 50Kg/cm². The PPS tack free film was peeled apart and the thermallyconductive sheet was processed into a predetermined shape and size. Thethermal conductivity, coefficient of thermal expansion, break downvoltage and flexural strength were measured. The results are shown inTable 6.

TABLE 6 Evaluation of Thermally conductive Substrate Break Experi-Thermal Thermal Down Flexural ment Conductivity Expansion VoltageStrength No (W/mK) (ppm/° C.) (kV/mm) (Kg/mm²) 1k 3.6 14 12 11.3 1l 3.713 14 13.5 1m 3.9 13 14 15.5 1n 4.1 15 15 17.8 1o 3.6 14 15 14.3 1p 3.913 15 18.9

As is apparent from Table 6, flexibility could be provided to thethermally conductive sheet by adding the resin that was liquid at roomtemperature. Moreover, the performance peculiar to inorganic filler wasexhibited. As compared with the method of the above mentioned Examplewhere a high boiling point solvent was added, the break down voltage byvoid and the flexural strength were excellent, because no solventexisted in the sheet at the time of molding the thermally conductivesheet.

EXAMPLE 2

In this Example, a thermally conductive substrate in which the thermallyconductive sheet was manufactured by the same method as in Example 1 andintegrated with a lead frame will be explained. The compositions of thethermally conductive sheet used in this Example will be describedhereinafter.

(1) Inorganic filler: 90 weight % of Al₂O₃, “AS-40®” (the product ofSHOWA DENKO K.K.) having a spherical shape and an average particle sizeof 12 μm.

(2) Thermosetting resin: 9 weight % of cyanate ester resin, “AroCy M30®”(the product of Asahi-Ciba CO., Ltd.)

(3) Solvent having a boiling point of not less than 150° C.: 0.5 weight% of butyl carbitol. (the first grade of chemical reagent of KantoChemical CO, Inc.).

(4) Other additives: 0.3 weight % of “Carbon Black” (the product ofToyo-carbon CO., Ltd.), and 0.2 wt. % of dispersing agent, “PLYSURFF-208F®” (the product of DAI-ICHI SEIYAKU KOGYO CO.,LTD.).

A thermally conductive sheet (a thickness was 770 μm) comprising theabove mentioned compositions was used. As the lead frame, a copper platehaving a thickness of 500 μm which was processed by the etching methodand further applied with nickel plating was piled up and thermallypressed at the temperature of 110° C. and pressed at the pressure of 60Kg/cm². By such a process, the thermally conductive sheet flowed intogaps of the lead frame and was filled to the surface of the lead frameto form a structure as shown in FIG. 2D. Then, the thermally conductivesheet with which the lead frame was integrated was heated by a drier at175° C. for one hour, and thermosetting resin of the thermallyconductive sheet was hardened. Such process could be conducted for ashort time by only conducting the molding at low temperature, and thehardening could be conducted as a whole after molding, so that masstreatment in a short time was realized as an entire process. Moreover,as shown in FIG. 2E, the outer circumference of the lead frame was cutand the bending of the terminal was conducted, to thus completely formthe thermally conductive substrate. Moreover, in the above, the moldingprocess and the hardening process were separately conducted. However, aseries of process from thermal molding with pressing to hardening couldbe continuously conducted.

When the thermal conductivity of the thermally conductive substrateobtained as above was evaluated, the value was 3.7 W/mK. Consequently,about 2 times as high a performance as that of a conventional injectionmolding method or metal substrate could be realized. Moreover, for theevaluation of reliability, a reflow test was conducted at the maximumtemperature of 260° C. for 10 seconds. At this time, there were noabnormalities at the interface between the substrate and lead frame,thus indicating a strong adhesion at the interference.

EXAMPLE 3

In this Example, a thermally conductive substrate will be explained,where the thermally conductive sheet was manufactured by the same methodas in Example 1 and both sides of the sheet had metallic foil wiringlayers and conductive resin composition was filled between the layers toelectrically connect the layers. The compositions of the thermallyconductive sheet used in this Example will be described hereinafter.

(1) Inorganic filler: 90 weight % of Al₂O₃, “AS-40®” (the product ofSHOWA DENKO K.K.) having a spherical shape and an average particle sizeof 12 μm.

(2) Thermosetting resin: 9 weight % of “NRV-1010®” (the product of JapanREC CO., Ltd.), a mixture comprising 60 weight parts of brominatedmultifunctional epoxy resin as a main agent, 39.5 weight parts ofbisphenol A nobolak resin as a hardener, and 0.5 weight parts ofimidazol as a hardening accelerator.

(3) Solvent having a boiling point of not less than 150° C.: 0.5 weight% of butyl carbitol (the first grade chemical reagent of Kanto ChemicalCo, Inc.).

(4) Other additives: 0.3 weight % of “Carbon Black” (the product ofToyo-carbon CO., Ltd., and 0.2 wt. % of coupling agent, “Plen-actKR-55®” (the product of AJINOMOTO CO., INC).

A thermally conductive sheet having the tack free film manufactured fromthe above mentioned compositions and was cut into a predetermined sizewith through holes having a diameter of 0.15 mm punched in it by the useof carbon dioxide laser. The through holes were equally spaced at apitch of 0.2 to 2 mm from the surface of the tack free film (FIG. 4B).

A conductive resin composition for filling via hole 403, containing 85wt. % of spherical shaped copper metal powder, 3 wt. % of bisphenol Aepoxy resin (Epikote 828, the product of Yuka-shell epoxy Co., Ltd.) asthe resin composition , 9 wt. % of glycidyl ester system epoxy resin(YD-171, the product of Tohto Kasei Co., Ltd.), and 3 wt. % of amineadducts hardener (MY-24, the product of AJINOMOTO CO., INC) were kneadedwith three rolls, and filled in the through holes by the screen printingmethod (FIG. 4C). After the polyethylene terephthalate film 401 wasremoved from the thermally conductive sheet to which the paste wasfilled, a copper foil having a thickness of 35 μm and a rough surface onone surface was adhered in a way in which the rough surface facing theside of the thermally conductive sheet. Subsequently, the thermallyconductive sheet was thermally pressed at the pressing temperature of180° C. and at the pressure of 50 kg/cm² for 60 minutes to form adouble-sided thermally conductive substrate (FIG. 4E).

By such a process, epoxy resin of the thermally conductive sheet washardened and a strong adhesion to the rough surface of the copper foilwas obtained. At the same time, epoxy resin in the conductive resincomposition 403 was also hardened and mechanically and electricallyconnected with both sides of the copper foil through the inner via holeconnection.

The copper foil of this double-sided copper plated board was etched bymeans of an etching technique and a double sided wiring substrate,having a circuit on which an electrode pattern and a wiring pattern witha diameter of 0.2 mm were formed on the inner via holes, was obtained.When the thermal conductivity and the coefficient of thermal expansionof the thermally conductive substrate manufactured by this method weremeasured, the thermal conductivity was 4.1 W/mK and the coefficient ofthermal expansion for the temperature ranges from room temperature to150° C. was 10 ppm/° C., thus, exhibiting excellent properties. The flipchip mounting of a semiconductor was conducted by using this thermallyconductive substrate. The method includes: forming Au bump on theelectrode of the semiconductor device by the conventional wire bondingmethod; applying adhesives containing Ag—Pd as the conductive materialson the top of this bump: bonding to the electrode pattern that wasformed on the double-sided thermally conductive substrate by the flipchip method in which the surface of the semiconductor device was faceddownward; hardening; and further mounting with molding resin. On thedouble-sided thermally conductive substrate the semiconductormanufactured as mentioned above was mounted, a reflow test was conducted20 times at a maximum temperature of 260° C. for 10 seconds. At thistime, the change in the electrical resistance value including that ofconnection between the substrate and semiconductor was very small. Thatis, the initial connecting resistance was 35 mΩ/bump and the connectingresistance after the test was 40 mΩ/bump.

In comparison, in a conventional glass epoxy substrate on which thethrough holes were provided at 2 mm intervals, the resistance at thebonding portion between the semiconductor and the substrate wasincreased, because the coefficient of thermal expansion of semiconductorwas different from that of the substrate, so that the reflow test endedat ten times. On the other hand, the substrate of the present inventionhas a coefficient of thermal expansion in the plane direction of thesubstrate that is similar to that of a semiconductor. Thus, the changein the resistance value as a function of the numbers of reflow tests wassmall.

EXAMPLE 4

In this Example, a thermally conductive substrate, wherein the thermallyconductive sheet was manufactured by the same method as in Example 1,both sides of the sheet have a metallic foil wiring layers and throughhole copper plating was filled between the layers to electricallyconnect the layers, will be explained. The compositions of the thermallyconductive sheet used in this Example will be described hereinafter.

(1) Inorganic filler: 87 weight % of Al₂O₃, “AM-28®” (the product ofSHOWA DENKO K.K.) having a spherical shape and an average particle sizeof 12 μm.

(2) Thermosetting resin: 11 weight % of phenol resin, “PhenoliteVH4150®” (the product of DAINIPPPON INK AND CHEMICALS, INC.)

(3) Solvent having a boiling point of not less than 150° C.: 1.5 weight% of ethyl carbitol (the first grade chemical reagent of Kanto ChemicalCo, Inc.).

(4) Other additives: 0.3 weight % of “Carbon Black” (the product ofToyo-carbon CO., Ltd.), and 0.2 wt. % of coupling agent, “Plen-act,KR-55®” (the product of AJINOMOTO CO., INC) After the tack free film waspeeled off from the thermally conductive sheet which was manufactured byusing the above mentioned compositions, this thermally conductive sheetwas cut into a predetermined size. A copper foil having a thickness of35 μm and a rough surface on one side was adhered to the thermallyconductive sheet in a way in which the rough surface was faced to theside of the thermally conductive sheet. Then this structure wasthermally pressed for 60 minutes at 180° C. and at the pressure of 50kg/cm² to form a double-sided thermally conductive substrate.

By such a process, phenol resin in the thermally conductive sheet washardened to form the strong adhesion between the rough surface of thecopper foil and the thermally conductive sheet. Processing throughholes, having a diameter of 0.3 mm and by using the drill was conductedon the thermally conductive substrate on which the copper foil wasadhered. Moreover, a 20 μm thick copper plating was applied to theentire surface including the through holes. The copper foil of thisdouble-sided copper plated thermally conductive substrate was etched byan etching technique, and thereby a double sided substrate, on which awiring pattern can be formed, was obtained (FIG. 5). The thermalconductivity and the coefficient of thermal expansion of the thermallyconductive substrate manufactured by this method were measured and thethermal conductivity and the coefficient of thermal expansion in thetemperature ranges from room temperature to 150° C. were formed to be2.8 W/mK and 18 ppm/°C., and thus, exhibiting excellent properties.

EXAMPLE 5

Here, an example of the multi-layered wiring thermally conductivesubstrate will be explained. A plurality of the thermally conductivesheets manufactured by the same method as in Example 1 were used. Wiringlayers were provided to the plurality of the layers of the thermallyconductive sheets and they were electrically connected to the thermallyconductive sheets by using a conductive resin composition. Thecomposition of the thermally conductive sheet used in this Example willbe described hereinafter.

(1) Inorganic filler: 92 weight % of Al₂O₃, “AM-28®” (the product ofSHOWA DENKO K.K.) having a spherical shape and an average particle sizeof 12 μm.

(2) Thermosetting resin: 7.3 weight % of cyanate ester resin. “BT2170®”(the product of the Mitsubishi Gas Chemical Company, Inc.)

(3) Solvent having a boiling point of not less than 150° C.: 0.2 weight% of ethyl carbitol (the first grade chemical reagent of Kanto ChemicalCO, Inc.).

(4) Other additives: 0.3 weight % of “Carbon Black” (the product ofToyo-carbon CO., Ltd.) and 0.2 wt. % of coupling agent “Plen-act,KR-55®” (the product of AJINOMOTO CO., INC).

A thermally conductive sheet 600 comprising the above mentionedcompositions and having a tack free film polyethylene terephthalate 601was used. From the side of the polyethylene terephthalate film, which ison one side of this thermally conductive sheet, through holes 602 havinga diameter of 0.15 mm were formed at an equal spaced pitch of 0.2 to 2mm by the use of a carbon dioxide laser (FIG. 6). A conductive resincomposition 603 containing 85 wt. % of copper metal spherical shapedpowder, 3 wt. % of bisphenol A epoxy resin (Epikote 828, the product ofYuka-shell epoxy Co., Ltd.) as the resin composition, 9 wt. % ofglycidyl ester system epoxy resin (YD-171, the product of Tohto KaseiCo., Ltd.) and 3 wt. % of amine adducts hardener (MY-24, the product ofAJINOMOTO CO., INC) as a hardener was kneaded by three rolls and filledin the through hole 602 by the screen printing method.

Moreover, the tack free film 601 was peeled apart. A conductive resincomposition for forming wiring pattern containing 80 wt. % of theneedle-like Ag powder, 10 wt. % of bisphenol A epoxy resin (Epikote 828,the product of Yuka-shell epoxy Co., Ltd.) as the resin composition, 2wt. % of amine adducts hardener (MY-24, the product of AJINOMOTO CO.,INC) as the hardener, and 8 wt. % of turpentine oil as a solvent, waskneaded by three rolls and filled to the portion where the tack freefilm 601 was peeled apart by the screen printing method (FIG. 6D). Twoother thermally conductive sheets on which wiring patterns were formedwere prepared by the similar process (FIG. 6F and 6G). In addition, bythe same method, a thermally conductive sheet, where the conductiveresin composition 603 was filled in the through holes 602 (FIG. 6E), wasprepared and piled up in a way in which the thermally conductive sheetwas made to be at the top by adjusting places as shown in FIG. 6H. Ontothe outer most layer, a copper foil of 18 μm thickness and having arough surface on one side. The laminate of this thermally conductivesheet was thermally pressed for 60 minutes at a temperature of 180° C.and a pressure of 50 Kg/cm² to form a multi-layered thermally conductivesubstrate.

The copper foil of the multi-layered thermally conductive substrate wasetched by an etching technique to form a wiring pattern. Since thismulti-layered thermally conductive substrate used copper foil for theouter most layer portion, mounting of parts by means of soldering waspossible. Moreover, on an inner layer, a wiring pattern was formed bythe screen printing method. A line having a width of about 50 μm andinner via holes could be formed by a conductive resin composition. Thus,a high density wiring was possible, which makes this multi-layeredthermally conductive substrate very promising as a substrate formounting high density electrical circuits. When the thermal conductivityand the coefficient of thermal expansion of the thermally conductivesubstrate manufactured by this method were measured, the thermalconductivity was 4.5 W/mK and the coefficient of thermal expansion inthe temperature range from room temperature to 150° C. was 8 ppm/° C.,thus, showing good results.

Then, similar to the above, by using the flip chip mounting of asemiconductor, the thermally conductive substrate was evaluated as amulti-chip module. The method includes: forming Au bump on the electrodeof the semiconductor device by the conventional wire bonding method;applying adhesives containing Ag—Pd as the conductive material on thetop of this bump; bonding to the electrode pattern formed on thethermally conductive substrate by a flip chip method in which thesurface of the semiconductor device was faced downward; hardening; andmounting with molding resin. On the thermally conductive substratesemiconductor was mounted, and a reflow test was conducted 20 times at amaximum temperature of 260° C. for 10 seconds. At this time, the changein the electrical resistance value including that of the bonding betweenthe substrate and semiconductor was recognized to be very stably small.That is, the initial connecting resistance of 34 mΩ/bump was onlychanged to 37 mΩ/bump after the test.

In addition, when the certain current was flowed to the mountedsemiconductor chip through the substrate of the present invention and 1W of heat was continuously generated, the change of the electricalresistance value including that of the bonding between substrate andsemiconductor was measured. In the substrate of the present invention,the change in the resistance value was insignificant in respective ofthe number of inner via holes.

Moreover, in the above mentioned Examples 1 to 5, copper and silverparticles were used as the conductive filler in the conductive resincomposition. However, in the present invention, the conductive particlesare not limited to copper particles and other metal particles can beused. In particular, when nickel is used, a high electric conductivityin the conductive portion can be maintained.

As mentioned above, the thermally conductive sheet of the presentinvention can be used for a thermally conductive substrate, where aninorganic filler can be added at a high filler content into athermosetting resin which is in the not-hardened state; the coefficientof thermal expansion in the plane direction is approximately the same asthat of a semiconductor; and the high thermal conductivity can beprovided. In the thermally conductive sheet of the present invention, ahigh boiling point solvent can be added or a thermosetting resin that isliquid at room temperature can be used. In the thermally conductivesheet of the present invention, an inorganic filler can be added at ahigh fiber content while flexibility of the thermosetting resin in thethermally conductive sheet is maintained in the not-hardened state, andmolding the thermally conductive sheet into a desired shape at lowtemperature and low pressure is possible. In addition, a substrate canbe made rigid by hardening the thermosetting resin by a thermalpressing. The thermally conductive substrate on which a semiconductorcan be simply and directly mounted can be obtained by the use of thisflexible, thermally conductive sheet. Furthermore, in a thermallyconductive sheet in which the above mentioned thermally conductive resinwas mixed with a thermosetting resin that is liquid at room temperaturethere existed no solvent in the sheet since the drying of the solvent ofnot more than 100° C. had already been completed. Therefore, when thissheet is heated and hardened, voids are not generated. Consequently, itsthermal conductivity is excellent and insulating property is alsoexcellent.

The thermally conductive substrate of the present invention can realizea thermally conductive substrate on which a semiconductor having thermalradiation property can directly be mounted by using the thermallyconductive sheets by piling up a lead frame, and hardening the thermallyconductive sheet by means of thermal pressing to integrate with the leadframe.

Moreover, the thermally conductive substrate of the present inventioncan realize a doubled-sided thermally conductive substrate having a highthermal conductivity. This structure permits electrical conductivity onboth sides by forming through holes in the thermally conductive sheet,and filling the thorough holes with a conductive resin composition.Then, metallic foil patterns can be formed on both sides of the sheet.

Moreover, the thermally conductive substrate of the present inventioncan realize a high thermally conductive double-sided substrate whichpermits electric conductivity by copper plating to the through holes ofthe third embodiment.

Moreover, the thermally conductive substrate of the present inventioncan realize a thermally conductive substrate which is a multi-layeredsubstrate having a multi-layered circuit structure in which a pluralityof the thermally conductive sheets are used, through holes to whichconductive resin composition is filled are formed, a wiring pattern isformed on one side of the thermally conductive sheet and a plurality ofthe thermally conductive sheets are piled up.

As mentioned above, since the thermally conductive substrate (thesubstrate having a single-sided wiring structure, double-sided wiringstructure, and multi-layered wiring structure) using the thermallyconductive sheet of the present invention can be filled with aninorganic filler at a high filler content, it has high thermalconductivity that cannot be obtained in the case where the usual printedcircuit board is used. Moreover, since the thermally conductive sheethas flexibility and can be molded and processed into any shape, thesubstrate can be manufactured by a simple process. This case ofmanufacturing is extremely effective from an industrial viewpoint.Furthermore, the hardened substrate is rigid and mechanically strong,and it has the thermal conductivity and the coefficient of thermalexpansion equal to that of a semiconductor. Therefore, the thermallyconductive substrate of the present invention is a promising materialfor using as a power circuit substrate, which will be increasingly usedin the future, or as a substrate for mounting a digital high speedsignal processing LSI in which there occurs a loss of high power. Inaddition, it is effective as a multi chip module (M&M) or chip sizepackage (SP) for mounting flip chip where the semiconductors aredirectly mounted.

Finally, it is understood that the invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. The embodiments disclosed in this applicationare to be considered in all respects as illustrative and notrestrictive, so that the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A thermally conductive substrate comprising athermally conductive sheet containing 70 to 95 weight parts of aninorganic filler and 5 to 30 weight parts a resin composition, and alead frame integrated with the thermally conductive sheet, wherein asurface of the lead frame and a surface of the thermally conductivesheet are disposed in substantially the same plane, and furthercomprising a metal substrate disposed on a surface of the thermallyconductive sheet opposite the surface that is coplanar with the leadframe.
 2. A thermally conductive substrate according to claim 1, havinga coefficient of thermal expansion from 8 ppm/° C. to 20 ppm/°C.
 3. Athermally conductive substrate according to claim 1, having a thermalconductivity from 1 W/mK to 10 W/mK.
 4. A thermally conductive substrateaccording to claim 1, further comprising a printed circuit board withtwo or more wiring layers integrated into the thermally conductivesubstrate in proximity to the surface of the lead frame that is coplanarwith the surface of the thermally conductive sheet.
 5. A thermallyconductive substrate according to claim 1, wherein the inorganic fillerhas an average particle diameter from 0.1 μm to 100 μm.
 6. A thermallyconductive substrate according to claim 1, wherein the resin compositioncomprises a resin selected from the group consisting of epoxy resin,phenol resin, and cyanate resin.
 7. A thermally conductive substrateaccording to claim 1, wherein the resin composition comprises abrominated multifunctional epoxy resin, a bisphenol A novolak resin, andan imidazole.
 8. A thermally conductive substrate according to claim 1,wherein the inorganic filler comprises a filler selected from the groupconsisting of Al₂O₃, MgO, BN and AlN.
 9. The thermally conductivesubstrate of claim 1, wherein a part of the tip of the lead frame isbent and the bent portion is an external electrode.
 10. The thermallyconductive substrate of claim 2, having electronic parts attachedthereto.
 11. A thermally conductive substrate comprising a thermallyconductive sheet containing 70 to 95 weight parts of an inorganic fillerand 5 to 30 weight parts a resin composition, and a lead frameintegrated with the thermally conductive sheet, wherein a surface of thelead frame and a surface of the thermally conductive sheet are disposedin substantially the same plane, and a part of the tip of the lead frameis bent and the bent portion is an external electrode.
 12. A thermallyconductive substrate according to claim 11, having a coefficient ofthermal expansion from 8 ppm/° C. to 20 ppm/° C.
 13. A thermallyconductive substrate according to claim 11, having a thermalconductivity from 1 W/mK to 10 W/mK.
 14. A thermally conductivesubstrate according to claim 11, further comprising a metal substratedisposed on a surface of the thermally conductive sheet opposite thesurface that is coplanar with the lead frame.
 15. A thermally conductivesubstrate according to claim 11, further comprising a printed circuitboard with two or more wiring layers integrated into the thermallyconductive substrate in proximity to the surface of the lead frame thatis coplanar with the surface of the thermally conductive sheet.
 16. Athermally conductive substrate according to claim 11, wherein theinorganic filler has an average particle diameter from 0.1 μm to 100 μm.17. A thermally conductive substrate according to claim 11, wherein theresin composition comprises a resin selected from the group consistingof epoxy resin, phenol resin, and cyanate resin.
 18. A thermallyconductive substrate according to claim 11, wherein the resincomposition comprises a brominated multifunctional epoxy resin, abisphenol A novolak resin, and an imidazole.
 19. A thermally conductivesubstrate according to claim 11, wherein the inorganic filler comprisesa filler selected from the group consisting of Al₂O₃, MgO, BN and AlN.20. The thermally conductive substrate of claim 12, having electronicparts attached thereto.